Blood gas analysis measures the pressure exerted by the gases that we inhale and exhale dissolved in blood. The pH of 7.40 is a perfect balance of acids and bases.
Arterial blood, which supplies each body organ's metabolic needs, is usually uniform throughout the body, whereas venous blood varies in composition depending on the size and activity of the tissue it has bathed. The greatest difference between arterial and venous blood is its oxygen content, but pH and carbon dioxide content also vary.
Because of PO2, PCO2 and pH are so transient in vivo, the patient must be prepared properly. For accuracy, the patient must be in a steady-state condition, attained by having the patient rest for 20 to 30 minutes. Although the PO2 and PCO2 achieve their levels in the blood in only about 10 minutes, hemoglobin takes 20 to 30 minutes to saturate to its fullest with oxygen. Exertion can cause the hemoglobin to desaturate, thus altering the results. Change in breathing patterns cause alterations in results in less than 1 minute. Examples of such situations include eating, experiencing pain, apprehension, breath holding, crying, coughing and exertion. Some situations including cardiac arrest, cardiac arrhythmias, respiratory arrest, atelecstasis (collapsed lung), and pulmonary emboli, effect a drastic change in a patient's condition. The sudden onset of these will require an arterial blood gas analysis immediately for the physician to evaluate accurately the patient's respiratory and metabolic status.
If the patient is breathing spontaneously, provide for his physical and mental comfort by quiet and reassuring talk and a comfortable position. The patient should be relaxed, lying comfortably in bed for at least 20 minutes. Meanwhile no changes should have occurred that would affect the blood gases. If the F102 has been changed, a 30 minute wait is recommended before taking a blood sample. Nurse will time specimen collection.
After collection, immediately check the syringe or other device for air bubbles and carefully expel any bubbles. A container with ice water or other suitable coolant, large enough to permit immersion of the entire barrel of the syringe should be prepared before the sample is obtained. As soon as the device with the blood sample has been securely closed, mixed, and labeled, it should be immersed in coolant. The label must remain legible after the immersion in coolant. The entire container with the coolant and the sample should be sent to the laboratory and analyzed as soon as possible.
Air bubbles can grossly alter PO2 values depending on the amount and size of the bubbles and the PO2 of the blood sample itself. The smaller the bubbles, the greater the surface area in contact with the blood, and the quicker the variation will occur. Small, pale, pink, frothy air bubbles are usually caused by an ill-fitting syringe-needle junction; a needle too small for the syringe; or pulling back too hard on the syringe. The blood gas sample must be anaerobic. If a sample just drawn contains a small air bubble, the bubble must be expelled within 20 seconds. The syringe should then be made air tight by embedding the needle in a stopper. Bending the needle does not make the syringe air tight and is very dangerous to anyone handling the sample.
Every person performing arterial punctures should be familiar with the dangers of the procedure and with precautions designed to prevent hazards to the patient or alterations in test results.
1. Hematoma Pressure must be applied to the site immediately after the needle has been withdrawn and maintained for a minimum of 5 minutes. To be certain that the flow of blood through the artery has not stopped; the pulse should be felt through the gauze while pressure is applied. Patients on anticoagulant therapy or those with liver disease may bleed longer and may tend to form hematomas or to bleed externally. An arterial puncture site is more likely to bleed than is a venipuncture site because of higher pressure in the arteries. With aging and certain disease states, the elasticity of tissue decreases and stopping the flow of blood after puncture becomes more difficult. Pressure bandages are not advisable for routine arterial punctures because they may cut off circulation if too tight or allow bleeding if too loose.
2. Thrombosis A thrombus (adherent clot) may form if the inner wall of the vessel is injured. Thrombi usually form over time when a needle is left in place; they rarely form as a result of a single arterial puncture. Thrombi may occur both in arteries and in veins; in arteries, they have more serious consequences because not all arteries have collateral vessels to provide adequate alternate blood supply. The safety of a particular site for an arterial puncture primarily depends on the presence of collateral vessels. An Allen Test should be performed to ensure adequate collateral circulation before selecting an artery as the puncture site.
3. Arteriospasm A transient reflex constricts of the artery in response to pain or other nervous stimuli, arteriospasm can be induced by needle stimulation from a shaky hand. When this occurs, obtaining blood may be impossible, even though the needle is properly located in the lumen of the artery. Arteriospasm may also result in temporarily impaired blood supply to the tissue served by the artery.